1. Field of the Invention
The present invention relates to a thermal shock resistant thermoelectric material. More particulary, it relates to a thermoelectric material comprising an alloy, a solid solution or an alloy and a solid solution consisting essentially of iron disilicide and a small amount of boron. Further, it also relates to a thermoelectric material comprising an alloy, a solid solution or an alloy and a solid solution consisting essentially of iron disilicide, a small amount of boron and a small amount of one element or more selected from the group consisting of elements of Groups IIB, IIIB, VB, VIA, VIB, VIIA and VIII in the periodic table of elements.
The thermoelectric material is used as a constituent member of a thermoelectric generator element (thermocouples) for direct conversion of heat into electricity.
The thermoelectric generator element for direct conversion of heat into electricity is basically made up of high temperature junctions and low temperature junctions, and legs of p-type thermoelectric material (semiconductor) and legs of n-type thermoelectric material (semiconductor).
The high temperature junction of the thermoelectric generator element can generally be made up by joining the leg end of the p-type thermoelectric material and the leg end of the n-type thermoelectric material with a metal plate. The high temperature junction may also be constructed as a p-n junction obtained by directly joining the leg ends of the p-type thermoelectric material and n-type thermoelectric material in order not to cause a lowering of thermal shock resistance of the thermoelectric materials. On the other hand, the low temperature junctions have lead wires connected to the respective leg ends of the p-type thermoelectric material and n-type thermoelectric material.
When the high temperature junctions, such as p-n junctions, are heated by a heat source, such as city gas flame or petroleum flame, or heated by contact with a high temperature substance, a temperature gradient occurs between the high temperature junctions and the low temperature junctions and thermoelectromotive force can be taken out from lead wires connected to the respective thermoelectric material leg ends of the low temperature junctions.
The thermoelectric generator element having the thermoelectromotive force (thermo. e.m.f.) can be utilized for compact power sources for safety valves of various gas apparatuses, or pairs of the thermoelectric generator elements may be combined to use for power sources for cordless hot air heaters using gas or petroleum as heat sources. This thermoelectric generator element can also be use as a sensor of temperature.
2. Description of Prior Art
The thermoelectric material should desirably be high in thermoelectromotive force obtained at the same time as being stabilized in air at high temperature and oxidation resistant.
Transition metal silicides are generally stabilized at high temperatures and are high temperature oxidation resistant. Because of this, they can be used as thermoelectric materials in air at high temperatures. Of these transition metal silicides, iron disilicide is stabilized at high temperatures and is high temperature oxidation resistant. It can be used in air even at 900.degree. C. Moreover it is also high in thermoelectromotive force. This thermoelectromotive force is as high as 10-50 times that of metal thermocouples, such as platinum-platinum rhodium thermocouples, chromel-alumel thermocouples, copper-constantan thermocouples and so on. Iron disilicide, however, was unsatisfactory in thermal shock resistance and entailed the disadvantage, for instance, that it was broken at one time when quenched in water, after being heated at one end at 900.degree. C.
Because of this, when cooling materials (such as water, oil and so on), such as water drops, become attached to the high temperature junctions of the thermoelectric generator element in the case of using a thermoelectric generator element containing iron disilicide as the constituent member at high temperatures, the high temperature junctions will break due to thermal shock.
Some doped iron disulfides are known. However, improving the thermal shock resistance of iron disilicide by addition of doping materials was heretofore unknown.
Thermoelectric materials obtained by doping iron disilicide with 2 to 5 weight % of cobalt or aluminum are known (P. M. Ware and D. J. McNeill, PROC. IEE, Vol. III, No. 1, January 1964, pp 178-182). The thermoelectric materials disclosed in this treatise contain no boron and are not thermally shock resistant.
A p-type thermoelectric element is known which is obtained by incorporating 0.03 to 0.25 mol%, based on iron disilicide, of transition metals excepting transition metals of Group VIII in the periodic table of elements (Japanese Patent No. 930733). No mention at all, however, is made in the instant patent of thermoelectric materials comprising boron-containing iron disilicide.
Further, a thermoelectric material comprising iron disilicide doped with manganese or cobalt is also known (I. Nishida, Phys. Rev. B, Vol. 7. No. 6, pp (2710-1713 (1973)). This thermoelectric material, however, contains no boron. Nor is it thermally shock resistant.
The above three references each disclose the addition of the doping material to iron disilicide, but none of them discloses the addition of boron or marked improvement of thermal shock resistance by the addition of boron.